Spin filter bottom spin valve head with continuous spacer exchange bias

ABSTRACT

A high performance specular free layer bottom spin valve is disclosed. This structure made up the following layers: NiCr/MnPt/CoFe/Ru/CoFe/Cu/free layer/Cu/Ta or TaO/Al 2 O 3 . A key feature is that the free layer is made of a very thin CoFe/NiFe composite layer. Experimental data confirming the effectiveness of this structure is provided, together with a method for manufacturing it and, additionally, its longitudinal bias leads.

This is a division of patent application Ser. No. 09/633,768, filingdate Aug. 7, 2000, now U.S. Pat. No. 6,517,896 Spin Filter Bottom SpinValve Headwith Continuous Spacer Exchange Bias, assigned to the sameassignee as the present invention.

FIELD OF THE INVENTION

The invention relates to the general field of GMR recording heads formagnetic disk systems with particular reference to design of the freelayer.

BACKGROUND OF THE INVENTION

Read-write heads for magnetic disk systems have undergone substantialdevelopment during the last few years. In particular, older systems inwhich a single device was used for both reading and writing, have givenway to configurations in which the two functions are performed bydifferent structures. An example of such a read-write head isschematically illustrated in FIG. 1. The magnetic field that ‘writes’ abit at the surface of recording medium 15 is generated by a flat coil,two of whose windings 14 can be seen in the figure. The magnetic fluxgenerated by the flat coil is concentrated within pole pieces 12 and 13which, while being connected at a point beyond the top edge of thefigure, are separated by small gap 16. Thus, most of the magnetic fluxgenerated by the flat coil passes across this gap with fringing fieldsextending out for a short distance where the field is still powerfulenough to magnetize a small portion of recoding medium 15.

The present invention is directed towards the design of read element 20which can be seen to be a thin slice of material located betweenmagnetic shields 11 and 12 (12 doing double duty as a pole piece, asjust discussed). The principle governing the operation of read sensor 20is the change of resistivity of certain materials in the presence of amagnetic field (magneto-resistance). Most magnetic materials exhibitanisotropic behavior in that they have a preferred direction alongwhich-they are most easily magnetized (known as the easy axis). Themagneto-resistance effect manifests itself as a decrease in resistivitywhen the material is magnetized in a direction perpendicular to the easyaxis, said decrease being reduced to zero when magnetization is alongthe easy axis. Thus, any magnetic field that changes the direction ofmagnetization in a magneto-resistive material can be detected as achange in resistance.

It is widely known that the magneto-resistance effect can besignificantly increased by means of a structure known as a spin valve.The resulting increase (known as Giant magneto-resistance or GMR)derives from the fact that electrons in a magnetized solid are subjectto significantly less scattering by the lattice when their ownmagnetization vectors (due to spin) are parallel (as opposed toanti-parallel) to the direction of magnetization of the solid as awhole.

The key elements of a spin valve structure are shown in FIG. 2. Inaddition to a seed layer 22 on a substrate 21 and a topmost cap layer27, these key elements are two magnetic layers 24 and 26, separated by anon-magnetic layer 25. The thickness of layer 25 is chosen so thatlayers 24 and 26 are sufficiently far apart for exchange effects to benegligible (i.e. the layers do not influence each other's magneticbehavior at the atomic level) but are close enough to be within the meanfree path of conduction electrons in the material. If, now, layers 24and 26 are magnetized in opposite directions and a current is passedthough them along the direction of magnetization (such as direction 28in the figure), half the electrons in each layer will be subject toincreased scattering while half will be unaffected (to a firstapproximation). Furthermore, only the unaffected electrons will havemean free paths long enough for them to have a high probability ofcrossing over from 24 to 26 (or vice versa). However, once theseelectrons ‘switch sides’, they are immediately subject to increasedscattering, thereby becoming unlikely to return to their original side,the overall result being a significant increase in the resistance of theentire structure.

In order to make use of the GMR effect, the direction of magnetizationof one of the layers 24 and 26 is permanently fixed, or pinned. In FIG.2 it is layer 24 that is pinned. Pinning is achieved by firstmagnetizing the layer (by depositing and/or annealing it in the presenceof a magnetic field) and then permanently maintaining the magnetizationwith an undercoat of a layer of antiferromagnetic material, or AFM,(layer 23 in the figure). Layer 26, by contrast, is a “free layer” whosedirection of magnetization can be readily changed by an external field(such as that associated with a bit at the surface 15 of a magneticdisk).

The structure shown in FIG. 2 is referred to as a bottom spin valvebecause the pinned layer is at the bottom. It is also possible to form a‘top spin valve’ structure where the pinned layer is deposited after thepinning layer.

Ultra-thin free layers as well as MR ratio are very effective to obtainhigh output spin valve GMR heads for over 30 Gb/in² magnetic recording.In general, magneto-resistive devices have a characteristic responsecurve such that their sensitivity initially increases with the appliedfield, then is constant with applied field, and then decreases to zerofor even higher fields. It is therefore standard to provide a biasingfield to keep the sensor operating in the linear range where it is alsoat its most sensitive. However, as the free layer thickness decreases,it becomes difficult to obtain a controllable bias point, high GMR ratioand good magnetic softness all at the same time. Syntheticantiferromagnets (SyAF) are known to reduce magneto-static fields in apinned layer, but a large bias point shift due to sense current fieldsremains a problem for practical use of an ultra-thin free layer. Toovercome this problem, the spin-filter spin valve (SFSV) was invented.

In a SFSV, the free layer is placed between the Cu spacer and anadditional high-conductance-layer (HCL). SFSV reduces sense currentfields in the free layer by shifting the sense current center toward thefree layer, resulting in a smaller bias point shift by sense currentfields. High GMR ratio is maintained even in the ultra-thin free layerbecause the HCL improves the mean free path of a spin-up electron whilemaintaining the mean free path difference between spin-up and spin-downelectrons.

As discussed earlier, spin valve GMR heads may be either top or bottomtypes. The GMR sensor track is defined by a patterned longitudinalbiasing layer in the form of two bias stripes. These are permanentlymagnetized in a direction parallel to the surface. Their purpose is toprevent the formation of multiple magnetic domains in the free layer.The most commonly used longitudinal bias for the bottom spin valve iswith contiguous (abutted) junction hard bias. The problem with theabutted junction is the existence of a “dead zone” at the sensor ends. AMR sensor track defined by continuous spacer exchange bias (similar tothat for the DSMR) does not have the “dead zone”. This may be criticalfor a very narrow track for ultra-high density recording application.

A routine search of the prior art was performed. The followingreferences of interest were found. U.S. Pat. No. 5,637,235 (Kim et al.)shows a SV with a capping layer. U.S. Pat. No. 5,896,252 (Kanai) shows aSV with a free magnetic layer composed of a CoFe and NiFe sublayerswhile U.S. Pat. No. 5,648,885 (Nishioka et al.) teaches a SV with CoFefree layer.

SUMMARY OF THE INVENTION

It has been an object of the present invention to provide a spin-filtersynthetic antiferromagnetic bottom spin valve that is suitable forultra-high density magnetic recording applications.

Another object of the invention has been to provide suitablelongitudinal biasing leads for this structure.

A further object of the invention has been to provide processes for themanufacture of these structures.

These objects have been achieved in a structure made up the followinglayers:

NiCr/MnPt/CoFe/Ru/CoFe/Cu/(free layer)/Cu/Ta or TaO. A key feature isthat the free layer is made of thin CoFe plus a CoFe/NiFe compositelayer in which CoFe is thinner than NiFe. Experimental data confirmingthe effectiveness of this structure is provided, together with a methodfor manufacturing it and the longitudinal bias leads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a read-write head for a magneticdisk system.

FIG. 2 shows the cross-sectional structure of a spin valve according tothe teachings of the prior art.

FIG. 3 shows the cross-sectional structure of a spin-filter spin valveaccording to the teachings of the present invention.

FIG. 4 illustrates how the structure of FIG. 3 is modified in order toapply longitudinal bias leads to it.

FIG. 5 shows the structure of FIG. 3 after longitudinal bias leads havebeen added to it.

FIG. 6 shows a plan view of the structure of which FIG. 5 is across-section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Relative to NiFe, sputter-etching of tantalum or tantalum oxide (TaO) is3 times slower. In the present invention, the Ta or TaO capping layer ofthe bottom spin valve can be removed by using a carbon tetrafluoridereactive ion etching (RIE) process. The process for sputter etching theunderlying Cu and partially etching of NiFe is controllable. Thesefactors cause our process for forming an ultra-thin free layer bottomspin valve to be suitable for manufacturing.

Advantages of the processes and structures that we will disclose belowinclude the following:

Bottom spin valves made by this invention have larger output signalamplitude.

The process for sputter-etching of the underlying Cu and (partially)etching NiFe for the continuous spacer exchange bias is controllable.

With the above design considerations in mind, we have worked out astructure and fabrication process to form a SF-SyAF bottom spin valvehead with a very thin free layer. The GMR sensor track is defined byusing a continuous exchange spacer bias.

Using the CVC GMR sputtering system, bottom SF-SyAF spin valves having:NiCr/MnPt/CoFe(I)/Ru/CoFe(2)/Cu/CoFe+NiFe(free layer)/Cu/Ta orTaO/configuration were made. Free layers of the present work are made ofa very thin CoFe/NiFe composite layer. TaO in the present structure isformed by first depositing a thin (e.g. 10-15 Å) Ta film on the NiFefree layer, and then oxidizing it by oxygen plasma ashing.

We now describe the process of the present invention for both spinvalves and read heads. In the course of this description, the structureof the present invention will also become clear.

Referring now to FIG. 3, the process for manufacturing a spin valvebegins with the provision of substrate 21 onto which there is depositedmagneto-resistance-enhancing seed layer 22. Pinning layer 33 is thendeposited onto layer 22. This pinning layer is between about 100 and 200Angstroms thick. Our preferred material has been MnPt but similarmaterials such as InMn, MnNi, ot MnPtPd could also have been used. Thisis followed by pinned layer 34, a synthetic antiferromagnetic materialthat is actually a laminate details not shown), deposited as follows:

-   -   first a layer of cobalt-iron, between about 12 and 25 Angstroms        thick, then a layer of ruthenium, between about 6 and 9        Angstroms thick, and last a second layer of cobalt-iron, between        about 12 and 25 Angstroms thick. It is a requirement that these        two cobalt-iron layers differ in thickness by between about 2        and 10 Angstroms.

Next, non-magnetic copper spacer layer 25, between about 18 and 25Angstroms thick, is deposited onto layer 34.

In a key feature of the invention, free layer 35 is then deposited. Thisfree layer is actually a composite of a cobalt-iron layer, having athickness between about 3 and 15 Angstroms and a nickel-iron layer thatis between about 10 and 35 Angstroms thick, the CoFe being depositedfirst.

Next, high conductance copper layer 36, between about 5 and 15 Angstromsthick, is deposited on free layer 35. This is followed by the depositionof a specular reflection layer of either tantalum that may be leftunchanged at a thickness between about 10 and 20 Angstroms or that isconverted to tantalum oxide layer 37 through plasma oxidation, asdiscussed earlier. This tantalum oxide layer has a thickness betweenabout 15 and 30 Angstroms. Then, capping layer of aluminum oxide 38,between about 100 and 300 Angstroms thick, is deposited on layer 37.

The process is then completed by annealing. This takes the form ofheating in the presence a magnetic field of between about 5,000 and10,000 Oe, in a transverse direction, at a temperature between about 250and 280° C. for between about 5 and 10 hours.

The process for manufacturing a read head begins with the provision of abottom spin valve structure that includes an ultra-thin specular freelayer as described immediately above. First, capping layer 38 is removedby wet etching, thereby uncovering tantalum or tantalum oxide layer 37onto which a layer of photoresist (comprising soluble underlayer 40 aand insoluble top layer 40 b), suitable for later lift-off, is appliedarid then patterned to define the shape of a pair of conductor leads.This can be seen in FIG. 4.

Then, all tantalum or tantalum oxide that is not protected byphotoresist is removed by reactive etching in carbon tetrafluoride. Thisresults in the uncovering of high conductance copper layer 15, whichlayer serves as an effective etch stop layer. Etching, bysputter-etching, then continues until all uncovered high conductancecopper 36 has been removed, as well as a certain amount of nickel ironfrom free layer 35. The removed nickel iron is then refilled with alayer of ferromagnetic material such as NiFe or CoFe, to a slightlygreater thickness than the removed material (because of some uncertaintyin the thickness control). This is followed by deposition of a layer ofantiferromagnetic material.

Continuing our reference to FIG. 4, biasing layer 41 is then depositedon layer 35 (i.e. the refilled nickel-iron) to a thickness between about100 and 150 Angstroms. The biasing layer may be either an exchange biaslayer made of manganese-platinum or a similar material such as InMn,MnNi, or MnPtPd. This is followed by deposition of a layer of conductivematerial 42. Our preferred material for the layer of conductive materialhas been Ta/Au/Ta, but similar materials, such as Cr/Rh/Cr could alsohave been used. It is deposited to a thickness between about 300 and 400Angstroms.

At this point the liftoff process is invoked so that all photoresist,together with all material on the resist's surface, is removed, givingthe structure the appearance shown in FIG. 5. A plan view, of which FIG.5 is a cross-section, is shown in FIG. 6.

The process is completed by annealing. This involves heating in thepresence a magnetic field of between about 100 and 200 Oe in thelongitudinal direction, at a temperature between about 250 and 280° C.for between about 2 and 5 hours.

Experimental Verification of the Invention:

For comparison purposes, SF-SyAF top spin valves having:NiCr/Cu/NiFe+CoFe (free layer)/Cu/CoFe1/Ru/CoFe2/MnPt/NiCrconfigurations with equivalent layer thicknesses were also made.

To characterize free layer anisotropy, free layer structures made of 55NiCr/20 Cu/2 CoFe-34 NiFe/15 Cu/TaO/Al₂0₃ and 55 NiCr/15 Cu/34 NiFe-2CoFe/20 Cu/NiCr, respectively (where all numbers are thicknesses inAngstroms), for the bottom and top SFSV were also studied.

After forming free layer and GMR stacks, the deposited structures werefirst given a standard 6000 Oe transverse field 280° C.-5 hrs annealing.The high field annealing set up the pinned layer direction. Afterremoving Al₂O₃ capping by wet etching, the GMR and the free layerstacks, were further given a low field (100 Oe) 250° C.-5 hrs annealingto reset the free layer in the sensor direction. This low fieldannealing was used to simulate the exchange bias annealing process.

Comparisons of the top and bottom spin valve free layer magneticproperties are illustrated in Table I.

TABLE I Free layer structure: 80.9% NiFe B_(s) H_(c) H_(k) R_(s) Dr/r Oeto close HA CZB55/Cu15/NiFe32/CoFe3/Cu20/CZB50 Top 0.28 10.23 15.8424.12 0.54 9 Oe CZB55/Cu20/CoFe3/NiFe32/Cu15/TaO Bottom 0.28 6.77 14.6725.85 0.65 4 Oe where B_(s) = magnetic moment, H_(c) = free layercoercivity (oe), H_(k) = anisotropy field (oe), and R_(s) = sheetresistance (ohm/sq.)

As illustrated in TABLE I, the free layer of the bottom spin valve showssofter magnetic properties (i.e. lower H_(c) and H_(k) than that of thetop spin valve. To close the hard axis (HA) loops for the free layers,applied longitudinal fields of 9 and 4 Oe are needed for the top and thebottom spin valve respectively.

Magnetic performance properties of the top and bottom SF-SyAF spinvalves are listed in TABLE II. For the top spin valve with (55 NiFe/5CoFe) free layer, GMR ratio (Dr/r)=9.54% and output amplitude (Dr)=1.20ohm/sq. Dr/r and Dr for the bottom spin valve are 10% higher. Also H_(c)and H_(k) are lower.

TABLE II Structure: (80.9% NiFe/MP43%-2mt) B_(s) H_(c) H_(e) H_(k) R_(s)Dr/r Dr FOM CZB55/Cu15/NiFe55/CoFe5/Cu20/CoFe23/Ru 1 0.52 8.47 16.2 9.9412.6 9.54 1.20 0.65 7.5/CoFe18/MP150/CZB30/Al₂O₃CZB55/MP150/CoFe18/Ru7.5/CoFe23/Cu20/ 2 0.51 5.34 13.5 6.77 12.7 10.51.33 0.73 CoFe5/NiFe55/Cu15/Ta10/OL/Al₂O₃CZB55/Cu15/NiFe34/CoFe2/Cu19/CoFe23/Ru 3 0.28 7.20 13.5 7.44 14.6 9.741.42 1.33 7.5/CoFe18/MP150/CZB30/Al₂O₃CZB55/MP150/CoFe18/Ru7.5/CoFe23/Cu20/ 4 0.29 6.05 4.56 2.20 15.5 10.71.66 1.45 CoFe2/NiFe34/Cu15/Ta10/OL/Al₂O₃CZB55/MP150/CoFe18/Ru7.5/CoFe23/Cu20/ 5 0.27 5.92 8.53 4.07 15.9 12.82.03 1.89 CoFe10/NiFe20/Cu10/Ta10/Al₂O₃ where B_(s) = magnetic moment,H_(c) = free layer coercivity (oe), H_(e) = inter-layer coupling field(oe) H_(k) = anisotropy field (oe), and R_(s) = sheet resistance(ohm/sq.)

For ultra-high density recording, the free layer of the bottom spinvalve is made of a very thin CoFe/NiFe composite layer having a magneticmoment equivalent to that of a 37 Å thick NiFe layer. See Cell 3 andCell 4/Cell 5, respectively, for the top and the bottom spin valves withultra-thin free layer. Figure-of-merit (FOM) for the (2 Å CoFe/34 ÅNiFe) spin valves is about 2× greater than that with (5 Å CoFe/55 ÅNiFe) free layer. The difference between Cell 4 and Cell 5, is that thecomposite free layer in cell 5 has a thicker CoFe component. The FOM forthe (10 Å CoFe/20 Å NiFe) spin valve with 1 Å Cu HCL is about 2.5×greater than that of the (5 ÅCoFe/55 Å NiFe) spin valve with 15 Å CduHCL. Besides having greater FOM, the bottom spin valve has shown softermagnetic properties than the top spin valve. These results indicate thata bottom spin valve head gives higher sensor sensitivity to yield evenhigher output signal.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A read head, comprising: a bottom spin valve that includes a specularfree layer, said free layer further comprising a cobalt-iron layer thatis between 5 and 8 Angstroms thick and a nickel-iron layer that isbetween 19 and 21 Angstroms thick; on the free layer, a high conductancecopper layer everywhere except in a lead area; on the high conductancecopper layer, a specular reflection layer; on the nickel-iron layer inthe lead area, an exchange biasing layer selected from the groupconsisting of MnPt, InMn, MnNi, and MnPtPd; and on said exchange biasinglayer, a layer of conductive material.
 2. The read head described inclaim 1 wherein said specular reflection layer is tantalum or tantalumoxide.